An electrical deionization apparatus is an apparatus in which concentration compartments and deionization compartments are formed by arranging cation exchange membranes and anion exchange membranes between electrodes, i.e. a cathode and an anode, and taking a potential gradient as a driving source, ions in water to be treated in the deionization compartments are made to migrate, and hence be separated out, through the ion exchange membranes into the concentration compartments, thus removing ionic components.
FIG. 1 shows a conceptual drawing of a typical conventional electrical deionization apparatus. With the electrical deionization apparatus shown in FIG. 1, anion exchange membranes A and cation exchange membranes C are arranged alternately between a cathode (−) and an anode (+), thus forming deionization compartments and concentration compartments. By further repeating the alternate arrangement of anion exchange membranes and cation exchange membranes, a plurality of deionization compartments and concentration compartments are formed alternately. If necessary, ion exchangers are packed into the deionization compartments and the concentration compartments to promote migration of ions through the compartments. Moreover, the compartments contacting the anode and the cathode at either end are generally referred to as the anode compartment and the cathode compartment, and fulfil a function of giving and receiving electrons of a current applied as a direct current.
During operation of such an electrical deionization apparatus, a voltage is applied between the anode and the cathode, and water is passed into the deionization compartments, the concentration compartments and the electrode compartments. Water to be treated having therein the ions to be subjected to treatment is fed into the deionization compartments, and water of a suitable water quality is passed into the concentration compartments and the electrode compartments. In FIG. 1, an example is shown in which RO treated water is fed into all of the deionization compartments, the concentration compartments and the electrode compartments. When water is passed into the deionization compartments and the concentration compartments in this way, in each of the deionization compartments the cations and anions in the water to be treated are attracted to the cathode side and the anode side respectively; because only anions permeate selectively through the anion exchange membranes and only cations permeate selectively through the cation exchange membranes, cations (Ca2+, Na+, Mg2+, H+ etc.) in the water to be treated pass through a cation exchange membrane C and migrate into a concentration compartment on the cathode side, and anions (Cl−, SO42−, HSiO32−, HCO3−, OH− etc.) pass through an anion exchange membrane A and migrate into a concentration compartment on the anode side. On the other hand, migration of anions from the concentration compartment on the cathode side into the deionization compartment and migration of cations from the concentration compartment on the anode side into the deionization compartment are blocked due to each of the ion exchange membranes having a property of blocking ions of the opposite sign. As a result, deionized water having a reduced ion concentration is obtained from the deionization compartments, and concentrated water having an increased ion concentration is obtained from the concentration compartment.
According to such an electrical deionization apparatus, upon using water containing few impurities on a level of, for example, RO (reverse osmosis membrane) treated water as the water to be treated, pure water of yet higher purity is obtained as the deionized water. Recently, there has come to be a demand for ultrapure water of yet higher purity, for example ultrapure water for semiconductor manufacture. With recent electrical deionization apparatuses, a method is thus known in which cation exchange resin beads and anion exchange resin beads are mixed together and packed as an ion exchanger into the deionization compartments and/or the concentration compartments and/or the electrode compartments so as to promote migration of ions in these compartments. Furthermore, there has also been proposed a method in which as an ion exchanger, a cation exchange fibrous material (nonwoven fabric etc.) on the cation exchange membrane side and an anion exchange fibrous material on the anion exchange membrane side are disposed facing one another in a deionization compartment, and a spacer or an ion-conducting spacer that has been made ion-conductive is packed between these ion exchange fibrous materials (see, for example, JP-A-H5-64726, and WO 99/48820).
When water to be treated is passed through a deionization compartment having an ion exchanger packed therein as described above, ion exchange groups in the ion exchanger and the salt to be removed in the water to be treated undergo an ion exchange reaction, whereby the salt is removed. For example, in the case of using NaCl as the salt to be removed, sulfonic acid groups as cation exchange groups and a quaternary ammonium salt as anion exchange groups, the following description applies.
When the water to be treated having the salt to be removed (NaCl) dissolved therein contacts the cation exchanger, the cations (Na+) in the water to be treated are subjected to ion exchange by the cation exchange groups, and are adsorbed onto the solid phase (cation exchanger) and thus removed (equation 1).NaCl+R—SO3−H+HCl+R—SO3−Na+  (1)
After contacting the cation exchanger so that the cations have been removed to some extent, the water to be treated next contacts the anion exchanger. At this time, the acid (HCl) that has been produced through the ion exchange reaction (equation 1) due to the cation exchange groups is completely neutralized as shown in equation 2.HCl+R—N+(CH3)3OH−→H2O+R—N+(CH3)3Cl−  (2)
On the other hand, salt to be removed in the water to be treated that has not reacted with the cation exchanger contacts the anion exchanger, and the anions (Cl−) are subjected to ion exchange by the anion exchange groups as shown in equation 3, and are adsorbed onto the solid phase (anion exchanger) and thus removed.NaCl+R—N+(CH3)3OH−NaOH+R—N+(CH3)3Cl−  (3)
Next, the water to be treated contacts the cation exchanger, and the alkali (NaOH) that has been produced through the ion exchange reaction due to the anion exchange groups (equation 3) is neutralized as shown in equation 4.NaOH+R—SO3−H+→H2O+R—SO3−Na+  (4)
Equations 1 and 3 above are equilibrium reactions, and hence the salt to be removed contained in the water to be treated is not completely removed by ion exchange upon contacting the anion exchanger and the cation exchanger once, but rather remains in the water to be treated to some extent. To remove the ions efficiently, it is thus necessary to carry out the reactions of equations 1 to 4 repeatedly, and hence it is important to make the water to be treated contact the cation exchanger and the anion exchanger alternately as many times as possible so as to make the salt to be removed migrate into the solid phase through the reactions of equations 1 to 4.
For the ions to be removed in the water to be treated to undergo ion exchange and neutralization reactions as described above, a two-stage process of the ions to be removed migrating into the vicinity of a functional group and then being subjected to the ion exchange reaction is required. In an electrical deionization apparatus, the water to be treated is fed continuously into each deionization compartment, and thus must undergo the ion exchange and neutralization reactions during the short time of passing through the deionization compartment; it is thus desirable for the ions to be removed in the water to be treated to diffuse into the vicinity of a functional group of an ion exchanger in a short time, and for the frequency of contact between the functional groups and ions to be kept high.
Moreover, in an electrical deionization apparatus, the ions to be removed that have been adsorbed onto the solid phase (ion exchangers) through the ion exchange and neutralization reactions of equations 1 to 4 above must be made to migrate from a deionization compartment into a concentration compartment or electrode compartment through electrical driving. Moreover, at this time, it is desirable for the ions to be removed that have been adsorbed onto the ion exchangers to migrate up to an ion exchange membrane between the deionization compartment and a concentration compartment continuing over the solid phase (ion exchanger) without desorbing out into the liquid phase. That is, in the deionization compartment, it is desirable for the cation exchanger contacting the cation exchange membrane and the anion exchanger contacting the anion exchange membrane to each be packed in so as to form a continuous phase between the cation exchange membrane and the anion exchange membrane.
Furthermore, in an electrical deionization apparatus in which ion exchangers are packed into compartments as described above, in each of the deionization compartments and/or concentration compartments having the ion exchangers packed therein, there exist sites where the cation exchange groups and anion exchange groups contact one another. At a site where a cation exchange group and an anion exchange group contact one another in a deionization compartment in particular, dissociation of water (equation 5) will occur under the steep potential gradientH2O→H++OH−  (5)and the ion exchangers will be regenerated in the deionization compartment through the H+ ions and OH− ions produced through this dissociation of water (water splitting), whereby pure water of high purity can be obtained. For efficient deionization, it is thus desirable to make it such that there are many sites where the water splitting occurs, i.e. many contact sites between the anion exchanger and the cation exchanger. Furthermore, the H+ ions and OH− ions produced through the dissociation of water continue through ion exchange groups of adjacent cation and anion exchangers one after another, bringing about regeneration. With this set-up, upon continuing the electrical driving, there will come to be a local lack of counter-ions to the functional groups at the contact sites between the cation exchanger and the anion exchanger, and water will dissociate in the vicinity of the functional groups so as to make up for the lacking counter-ions, whereby H+ ions and OH− ions can be continuously supplied to the cation exchange groups and the anion exchange groups. Moreover, it is thought that not only with water but also with a non-electrolyte such as an alcohol, polarization and dissociation will occur under a strong electric field to form anions and cations which will be adsorbed onto the functional groups, enabling removal. It is thus desirable for the contact sites between the anion exchanger and the cation exchanger (the sites where water splitting occurs) to be numerous and to be dispersed throughout the whole of each deionization compartment in particular, and furthermore it is desirable for the anion exchanger and the cation exchanger to each be disposed so as to form a continuous phase from the contact sites.
Furthermore, in recent years there has been a demand for pure water of yet higher purity, it being desired for the concentration of TOC (total organic carbon) components contained in treated water to be low. As TOC components contained in treated water obtained through electrical deionization treatment, there are those of internal origin, i.e. those originating from components leaching out from the ion exchangers packed into the deionization apparatus, and those of external origin, i.e. those originating from TOC contained in the water to be treated. Of these, many of the TOC components leaching out from the ion exchangers are unreacted monomers or uncrosslinked polymer electrolytes that became attached to an ion exchanger during synthesis of the ion exchanger. These gradually leach out into the liquid phase upon washing by passing water through, and it is desirable to make the ion exchangers have a structure such that this washing can be carried out in as short a time as possible. Moreover, it is desirable to eliminate a crosslinking reaction from the ion exchanger synthesis process so that contamination of the ion exchangers with uncrosslinked polymer electrolytes can be prevented. Regarding TOC components contained in the water to be treated, on the other hand, these can be removed by being ionized under the strong potential gradient between cation and anion exchange groups as with the water dissociation reaction. It is thus desirable for it to be possible to pass water to be treated containing TOC components uniformly past the contact sites between the cation exchanger and the anion exchanger.
Moreover, it is further desirable for the treated water (pure water) obtained to have a low concentration of weak electrolytes such as silica and carbonate. Again, ionization of such weak electrolytes under the strong potential gradient between cation and anion exchange groups as with the water dissociation reaction is effective. In this case, it is thus again desirable for it to be possible to pass water to be treated containing weak electrolytes uniformly past the contact sites between the cation exchanger and the anion exchanger.
The functions demanded of an electrical deionization apparatus have been listed above; however, with electrical deionization apparatuses having a conventional constitution, it has not been possible to obtain an apparatus that satisfies all of these demands.
For example, with many conventional electrical deionization apparatuses, anion exchange resin beads and cation exchange resin beads have been packed mixed together in a deionization compartment. In this case, the state of packing of the resins is random, and moreover the flow of water through the compartment is also random, and hence looking microscopically, regarding the contact between the water to be treated and the ion exchangers, it has not necessarily been the case that the water to be treated contacts the anion exchanger and the cation exchanger alternately. Moreover, regarding the particle diameter of the ion exchange resin beads packed in, in general it is normal to use beads having a particle diameter of approximately 500 μm so as to keep the pressure loss down, but most of the functional groups of such ion exchange resin beads are present in macropores and micropores inside the beads, and hence it is difficult for ions to be removed to diffuse up to the vicinity of a functional group, and hence the frequency of contact between the ions to be removed and the functional groups is not very high. Moreover, because the cation exchanger and the anion exchanger are packed in randomly, it is difficult for each of the cation exchanger and the anion exchanger to form a continuous phase, and hence it is difficult for the ions to be removed to migrate continuously through the solid phase from the deionization compartment into a concentration compartment, and moreover the TOC component removal performance and the weak electrolyte removal performance are poor. Furthermore, there has been a problem that there is much leaching out of TOC components from the ion exchange resin beads, and in particular it is difficult to completely remove TOC components leaching out from the inside of macropores and micropores even if the resin beads are washed by passing water through for a long time.
Moreover, among conventional electrical deionization apparatuses, ones in which ion exchange resin beads are packed in layers have also been proposed. As conventional electrical deionization apparatuses of this form, there are ones in which anion exchange resin beads and cation exchange resin beads are packed in a deionization compartment alternately with a plastic mesh screen or the like interposed therebetween as required, ones in which a deionization compartment is partitioned with partitions and anion exchange resin bead-only beds and cation exchange resin bead-only beds are formed alternately in the compartments produced through the partitioning, ones in which blocks are formed by binding ion exchange resin beads together with a binder and anion exchange resin bead blocks and cation exchange resin bead blocks are packed in alternately, and so on. However, packing anion exchange resin beads and cation exchange resin beads into a deionization compartment in orderly fashion while forming alternate layers is very difficult. Moreover, in the case that a mesh screen or the like is interposed between layers, or the deionization compartment is partitioned with partitions and layers are formed alternately, the contact sites between the anion and cation exchange groups (the sites where water splitting occurs) are limited to the planes of contact between the ion exchange membranes and the ion exchangers packed into the compartment, and hence it is not possible to form a large number of sites where water splitting occurs in the deionization compartment. Moreover, in the case that the deionization compartment is partitioned with partitions, the number of resin layers that can be formed is limited to from a few to a few tens from the standpoint of the ease of assembling the compartment and the overall size of the apparatus. Moreover, if ion exchange resin beads are used, then as described above, the frequency of contact between the ions to be removed and the ion exchange groups is not very high. Furthermore, in the case of binding ion exchange resin beads together with a binder, the flow path of the water is restricted by the binder, and hence the frequency of contact between the ions to be removed and the ion exchange groups drops markedly. Moreover, because ion exchange resin beads are used, as described above, the amount of TOC leaching out is high, and in particular in the case of using a binder, the binder itself becomes a component that leaches out, and hence the TOC concentration in the treated water becomes yet higher. Furthermore, as described above, the contact sites between the anion and cation exchange groups are limited to the planes of contact between the ion exchange membranes and the ion exchangers packed into the compartment, and most of the water to be treated does not flow through here, and hence ionization of TOC components and weak electrolytes is difficult, and thus the removal performance thereof is poor.
To resolve the various problems associated with using ion exchange resin beads described above, using an ion exchange fibrous material obtained by introducing ion exchange groups onto a fibrous material such as a woven fabric or a nonwoven fabric by radiation-induced graft polymerization or the like as a material packed into a deionization compartment has been proposed (e.g. previously mentioned JP-A-H5-64726). Such an ion exchange fibrous material has a greater specific surface area than ion exchange resin beads, and it is not the case that the ion exchange groups are present in micropores or macropores inside beads as with ion exchange resin beads, and hence a very large number of ion exchange groups can be disposed on the fiber surfaces. The ions to be removed in the water to be treated are thus readily transported by the flow (by convection) to the vicinity of the ion exchange groups. If an ion exchange fibrous material is used, then compared with the case of using ion exchange resin beads, the frequency of contact between the ions to be removed and the ion exchange groups can thus be increased markedly.
However, a fibrous material such as woven fabric or a nonwoven fabric does not generally have a very high water permeability, and hence it has been thought that if a fibrous material is packed into a conventional thin deionization cell, then the pressure loss will be too high, and hence it will not be possible to obtain a sufficient treatment flow rate.
An electrical deionization apparatus has thus been proposed in which a cation exchange fibrous material such as a nonwoven fabric on the cation exchange membrane side and an anion exchange fibrous material on the anion exchange membrane side are disposed facing one another in a deionization compartment, and for an example an oblique net-shaped spacer, or an ion-conducting spacer obtained by making such a spacer ion-conductive, is packed between these ion exchange fibrous materials (e.g. the previously mentioned WO99/48820). In the case of an apparatus having such a constitution, the water to be treated becomes turbulent inside the oblique net-shaped spacer or ion-conducting spacer, and contacts the cation exchange fibrous material and anion exchange fibrous material. Although the water to be treated thus contacts the cation exchange fibrous material and the anion exchange fibrous material alternately to some extent, one cannot say that alternate contact is carried out sufficiently efficiently with this constitution. Moreover, although fibrous materials having a large surface area and many usable ion exchange groups are used, due to the difference in the water permeability between the fibrous materials and the spacer, most of the water to be treated flows through the spacer part, and very little flows through the inside of the nonwoven fabrics. The frequency of contact between the ions to be removed and the ion exchange groups is thus low.